Plasma melting and deposition of particulate materials, be it ceramic or metallic powders has been known and used on an industrial scale since the late 60's and early 70's. Industrial plasma spraying devices are mostly of the DC type where an electric arc is established between a pair of electrodes to ionize a gas injected into the annular space between the electrodes. The body of plasma reaches very high temperatures, sufficient to melt the particulate material.
A common feature of the prior art devices is that the particulate material to be treated is injected in the plasma where it is heated, molten and accelerated to a relatively high velocity before impinging on the substrate on which the particulate material is to be deposited. The maximum velocity and temperature attained by the particles are limited by the velocity and the volume of the plasma body. DC plasma devices, giving rise to high velocity flows of the order of 100 to 300 m/s, are inherently small volume plasmas and can operate only at a small deposition rate. Therefore, these devices are ill suited for applications requiring high deposition rates.
An alternative to the DC plasma spraying device is the inductively coupled plasma apparatus which uses a radio frequency inductor coil for coupling energy into the plasma gas, instead of using electrodes. Inductively coupled plasmas are large volume plasmas, however, they give rise only to low gas velocities, of the order of 20 to 30 m/s.
An object of the present invention is an inductively coupled plasma apparatus for heating and depositing particulate material in which the particles travel at high velocities.
The object of the invention is achieved by providing an inductively coupled plasma torch in which the particles to be deposited are accelerated at a velocity higher than the velocity of the plasma gas flowing in the container, preferably of the order of 100 m/s or more, prior to their injection into the plasma body. The particles are injected in a low velocity, large volume induction plasma where they are heated and molten without much loss of their initial inertia and velocity.
In a preferred embodiment, the particles of material to be deposited are accelerated through viscous drag with a carrier gas traveling at a high velocity in a feed line leading to the plasma container. The carrier gas and the particles of material are injected in the plasma container, upstream of the body of plasma, in a direction generally parallel to the flow of plasma gas therein so that the particles pass through the body of plasma in the container, are heated, and then impinge on the substrate.
To prevent the local cooling and instability of the plasma which may be caused by the carrier gas injected at high velocity in the plasma container, the velocity of the carrier gas is reduced before the injection thereof in the plasma container. The velocity reduction is carried out by expanding the carrier gas in volume at the nozzle of the feed line. The expansion is performed suddenly, immediately before the carrier gas enters the plasma container to limit the residence time of the particulate material into a mass of low velocity carrier gas in the feed line nozzle, thus preventing a substantial reduction of the particles velocity.
The apparatus and the method, according to the present invention, find wide applications in the areas of deposition of metal, alloys and ceramic powders, remelting, titanium sponge melting as well as the forming of refractory ceramics and high purity materials, among others.
The present invention comprises, in a general aspect, a process for heating and depositing a particulate material on a substrate, the process comprising the steps of:
flowing ionizable plasma gas at a certain velocity in a plasma container along a longitudinal axis thereof;
inductively coupling energy to the plasma gas to create in the plasma container a body of plasma directed toward the substrate;
accelerating the particulate material to be deposited on the substrate to a velocity higher than the velocity of the plasma gas flowing in the plasma container; and
feeding the particulate material in the plasma container along a longitudinal axis thereof, wherein the particulate material is heated while passing in the body of plasma at a velocity higher than the velocity of the plasma gas and is deposited on the substrate.
The invention also comprehends an apparatus for heating and depositing a particulate material on a substrate, the apparatus comprising;
a plasma container having an open end facing the substrate;
first inlet means on the plasma container to supply ionizable plasma gas at a certain velocity in the plasma container flowing along a longitudinal axis thereof;
inductor means mounted on the plasma container for coupling energy to the plasma gas to sustain a body of plasma in the plasma container;
particulate material supply means communicating with the container for supplying therein the particulate material along a longitudinal axis thereof, the particulate material supply means comprising means for accelerating the particulate material at a velocity higher than the velocity of the plasma gas in the plasma container.